1. Field of the Invention
This invention generally relates to electrochemical cells and, more particularly, to an anode preloaded with consumable metals, and an associated fabrication process.
2. Description of the Related Art
The rechargeable lithium ion battery (LIB) has triggered the portable electronic devices revolution due to its high power density, long cycling life, and environmental compatibility. The rechargeable LIB consists of a cathode (positive electrode) and an anode (negative electrode), separated by a Li+-ion permeable membrane. A solution or polymer containing lithium-ions is also used in the battery so that Li+-ions can “rock” back and forth between the positive and negative electrode freely. The positive materials are typically transition-metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), lithium iron phosphate (LiFePO4), and their derivatives. Lithium-ions can move in their interstitial space freely and reversibly. The negative electrode materials can use lithium-metal, alloys, and carbonaceous materials. During discharge, Li+-ions are extracted from the negative electrode and inserted into the positive electrode. In the meantime, electrons pass through an external circuit from the negative electrode to the positive electrode and generate electric power. During a charge, ions and electrons move along the reverse direction and go back to their original places.
Although LIBs have been successfully used, the conflict between lithium demand and its scarcity surges its cost, which hinders the further application of lithium-ion batteries on a large scale. Therefore, a low-cost rechargeable battery is urgently needed as an alternative to expensive LIBs. Under the circumstance, sodium-ion batteries are attracting more attention because sodium has very similar properties to lithium, but a cheaper cost. Like lithium-ion batteries, sodium-ion batteries need Na+-host materials as their electrode. Much effort has been expended to directly duplicate the Li+-host structures, using Na+-host electrode materials for the sodium-ion batteries. For example, NaCoO2, NaMnO2, NaCrO2 and Na0.85Li0.17Ni0.21Mn0.64O2, all having a layered-structure similar to LiCoO2, have been developed for sodium-ion batteries. Similarly, Co3O4 with a Spinel structure, Na3V2(PO4)3 with a NASICON structure, and NaFePO4 with an Olivine structure have been employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F and Na1.5VOPO4F0.5, have also used as the positive electrode in sodium batteries.
However, it is impractical to copy the structures of Li+-host compounds for Na+ or K+-host compounds. Sodium and potassium ions are much larger than lithium ions, and severely distort the structure of the Li+-host compounds. Thus, it is very important for the advancement of sodium/potassium-ion batteries to develop new Na+/K+-host materials with large interstitial spaces in which sodium/potassium-ions can easily and reversibly move. Na+/K+-ions have been observed to intercalate into metal cyanide compounds.
FIG. 1 is a diagram depicting the crystal structure of a metal hexacyanometallate (MHCM) (prior art). MHCMs can be expressed as AXM1YM2Z(CN)N.MH2O, where A can be selected from, but not limited to alkali and alkaline metals, and M1 and M2 are transition metals such as titanium (Ti), vanadium (V), chrome (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), calcium (Ca), magnesium (Mg), etc. M1 and M2 can be the same or a different metal. The ratio (X:N) of M1 and M2 is a variable, depending on the materials used. Transition metal hexacyanoferrates (TMHCFs) with large interstitial spaces have also been investigated as cathode materials for rechargeable batteries. TMHCM can be expressed as M1YM2Z(CN)N.MH2O.
Unlike cathode material, for which various materials are available for both LIBs and MeIBs, where Me is a metal such as Na, potassium (K), magnesium (Mg), calcium (Ca), cesium (Cs), or aluminum (Al), options for anode material are very limited, especially for MeIBs. Currently, graphite is the most widely used anode material for LIBs. Its layered structure allows lithium intercalation between carbon layers, and the reaction can be written as follows:Li++C6+e→LiC6 
Such an intercalation reaction occurs below 0.2 V vs. Li+/Li and has good reversibility, resulting in about 12% initial irreversible capacity in the first cycle. However, noteworthy is the fact that non-lithium Mea+ cannot insert into graphite layers because it has a larger size than Li+. As a result, alternative anode materials are highly desired in development of rechargeable MeIBs. Taking the anode materials for SIBs as an example, hard carbon or so called “non-graphitizable” carbonaceous materials is one candidate. Unlike graphite, which has poor large Me-ion (e.g. sodium) storage properties, it can sustain a capacity as high as 300 milliamp hours per gram (mAh/g), thanks to pseudocapacitive Me-ion absorption on its surface. Dahn's group prepared various hard carbon anodes derived from different low-cost organic precursors and evaluated their anodic properties in both LIBs and SIBS [1-2]. They also revealed the sodium insertion mechanism in hard carbon anodes [3-4]. Besides an intercalation process that occurs at a high potential range, corresponding to sodium insertion into graphene layers, filling in micro-pores on hard carbon surface was found at a low-potential range close to sodium metal. These reactions are only partially reversible in the first cycle, therefore resulting in a large irreversible capacity of more than 30%. Other anode materials, such as sodium-intercalation oxides, alloys and organic compounds, have been investigated for potential applications in SIBs. Li4Ti5O12, a zero-strain anode material for LIBs, was found to undergo a three-phase reaction during sodium intercalation [5]. It showed a reversible capacity of 150 mAh/g after an activation process in the first 20 cycles. Nevertheless, its initial coulombic efficiency was only 81%. Tarascon's group reported Na2Ti3O7 as a low-voltage anode that has the lowest desodiation potential among non-carbon sodium-intercalation compounds. However, its irreversible capacity is more than 40% [6]. Metals such as tin, antimony, and lead that can form alloys with sodium have a high reversible capacity (>500 mAh/g), however, they also show an initial capacity loss around 20% in the very first cycle [7, 8]. Recently, Hong et al. and Hu et al. demonstrated a reversible Na+ insertion behavior into an organic anode, Na2C8H4O4, which showed a reversible capacity of 300 mAh/g in a low voltage range [9, 10]. Like other anode materials, this organic anode shows poor reversibility in the first cycle. Aside from the use of these anode materials in SIBs, it is believed that they may also show a large irreversible capacity during the first cycle with other metal batteries because of similar electrochemical reactions. Since the irreversible reactions, i.e. solid electrolyte interface (SEI) formation, on the anode side consume metal-ions from the cathode, the result is a dramatic energy decrease in the full-battery. Therefore, the goal of reducing the irreversible capacity of these anodes becomes a major challenge in development of MeIBs.
It would be advantageous if an anode could be fabricated in a preloaded condition, so that it maintained a large reversible capacity when used in a battery.    [1] D. A. Stevens, J. R. Dahn, High Capacity Anode Materials for Rechargeable Sodium-ion Batteries, J. Electrochem, Soc. 147 (2000) 1271,    [2] Edward Buiel, J. R. Dahn, Reduction of The Irreversible Capacity in. Hard-Carbon. Anode Materials Prepared from Sucrose for Li-ion Batteries, J. Electrochem. Soc. 145 (1998) 1977.    [3] D. A. Stevens, J. R. Dahn, The Mechanism of Lithium and Sodium Insertion in Carbon. Materials, J. Electrochem. Soc. 148 (2001) A803,    [4] D. A. Stevens, J. R. Dahn. An in-situ Small-Angle X-ray Scattering Study of Sodium Insertion into A Nanoporous Carbon Anode Material within. An Operating Electrochemical Cell, J. Electrochem. Soc. 147 (2000) 4428,    [5] Yang Sun, Liang Zhao, Huilin. Pan, Xia Lu, Lin Gu, Yong-Sheng Hu, Hong Li, Michel Armand, Yuichi Ikuhara, Liquan Chen, Xuejie Huang, Direct Atomic-scale Confirmation of Three-phase Storage Mechanism in Li4Ti5O12 Anodes for Room-temperature Sodium-ion Batteries, Nat. Comm. 4 (2013) 1870.    [6] P. Senguttuvan, G. Rousse, V. Seznec, J. M. Tarascon, M. R. Palacin, Na2Ti3O7: Lowest Voltage Ever Reported. Oxide Insertion. Electrode for Sodium Ion. Batteries, Chem. Mater. 23 (2011) 4109.    [7] M. K. Datta, R. Epur, P. Saha, K. Kadakia, S. K. Park, P. N. Kumta, Tin and graphite based nanocomposites: Potential anode for sodium ion batteries, J. Power Sources, 225(2013) 316.    [8] Y. Zhu, X. Han, Y. Xu, Y. Liu, S. Zheng, K. Xu, L. Hu, C. Wang, Electrospun SbiC Fibers for a Stable and Fast Sodium-Ion Battery Anode, ACS Nano, 7 (2013) 6378.    [9] Y. Park, D. S. Shin, S. H. Woo, N. S. Choi, K. H. Shin, S. M. Oh, K. T. Lee, S. Y. Hong, Sodium terephthalate as an organic anode material for sodium ion batteries, Adv. Mater., 24(2012) 3562.    [10] A. Abouimrane, W. Weng, H. Eltayeb, Y. Cui, J. Niklas, O. Poluektov, K. Amine, Sodium insertion in carboxylate based materials and their application in 3.6 V full sodium cells, Energy Environ. Sci., 5(2012) 9632.    [11] M. E. Leonova, I. K. Bdikin, S. A. Kulinich, O. K. Gulish, L. G. Sevast'yanova, K. P. Burdina, High-Pressure Phase Transition of Hexagonal Alkali Pnictides, Inorg. Mater, 39 (2003) 266.